In producing a transparent silica glass product with high precision having a relatively complicated shape and made of a silica glass or a bubble-containing opaque silica glass product, there have hitherto been employed a method for welding a member having a simple shape by fire working and assembling it; a method for working a silica glass bulk body by employing a mechanical working technology; a method for carrying out cast molding; a method for carrying out press molding under reduced pressure; a method for thermally foaming ammonified amorphous silica; and the like. However, in the foregoing assembling method by welding, there were involved problems that almost all of steps are manually carried out so that very experience and skill and time are required; and further that sag is generated in end parts because of a high temperature generated at the time of fire working, whereby the appearance is hindered.
Also, in the mechanical working technology, cutting working using a grindstone or the like is carried out. Since the silica glass is a brittle material with high hardness, there were involved problems that it takes a very long time; and breakage, strain, distortion, and the like are frequently generated, thereby reducing a yield of a final product and shortening a life of a product. Furthermore, in the cast molding method, there were involved problems that since a quartz glass exhibits a high viscosity as 5 or more in terms of (log η) even at 2,000° C., an apparatus to be used at the time of baking is expensive; that even by using a carbonaceous material, it reacts with the silica glass at a high temperature and is remarkably consumed, it must be frequently exchanged; and further that the surface state of the resulting product is so bad that the surface must be largely removed by cutting or polishing working, resulting in an increase of the costs. In the method for carrying out press molding, the same problems were caused, too. In particular, in producing an opaque silica glass product, since a bubble size of the opaque silica glass replies upon a particle gap at the time of molding, namely particle size distribution or filling density, according to the foregoing production method, it was difficult to control the bubble size, whereby an opaque silica glass product having a relatively complicated shape and having a desired bubble size could not be obtained.
As a method for overcoming the foregoing problems and producing a silica glass product having a relatively complicated shape at a low price in a high purity with good precision, there is proposed an injection molding method described in, for example, Patent Document 1. Also, as a production example of an opaque silica glass product, there is enumerated a method described in, for example, Patent Document 2. According to the production method described in the foregoing patent document 1, a silica glass powder and a binder and a plasticizer which is added as the need arises are kneaded, and the mixture is injection molded under heating, degreased and vitrified to produce a silica glass product. However, there were involved problems that the amount of the binder to be used is large as from 10 to 70 parts by mass based on 100 parts by mass of the silica glass powder and shrinkage of a silica glass molded article occurs at the time of baking, whereby the generation of deformation or a crack is easy to occur; and that since the injection molding is carried out at a high temperature, the binder degenerates during molding and remains in the silica glass molded article, thereby contaminating the silica glass product. Also, according to the production method described in the foregoing Patent Document 2, there were involved drawbacks that an opaque silica glass having a desired bubble size is not obtained; and that the resulting product is limited to a molded article in an ingot form, a lot of steps are required for the purpose of obtaining a product having a complicated shape therefrom, and the production costs are high.
Patent Document 1: JP-A-4-349130, JP-A-2004-203639
Patent Document 2: JP-A-4-65328, JP-A-10-29836
The invention is concerned with a process for producing a silica glass product which has solved the foregoing problems; and a first object thereof is to provide a process for producing a transparent silica glass product having a relatively complicated shape in a high purity with high precision at a low price.
Also, a second object of the invention is to provide a process for producing an opaque silica glass product having a desired bubble size and having a relatively complicated shape in a high purity at a low price.